arXiv 0901.05142009-01-06astro-ph.CO

高红移低金属含量尘埃气体云中HD和H2的形成

HD and H2 formation in low-metallicity dusty gas clouds at high redshift

S. Cazaux, M. Spaans

备注：: typos corrected, accepted for publication in Astronomy and Astrophysics
DOI：: 10.1051/0004-6361:200811302

摘要：

背景：HD和H2分子在高红移的原始和贫金属气体的冷却中起着重要的作用。目的：研究了HD和H2的颗粒表面和气相形成，以评估微量尘埃10^{-5}-10 ^{-3} Zo在HD和H2产生中的重要性。研究方法：我们认为碳质和硅酸盐颗粒，包括物理吸附和化学吸附，隧道，和现实的粮食表面障碍。我们发现，对于一个耦合化学和热平衡的塌陷气体云环境，小到10^{-5}太阳的尘埃丰度会导致由于表面反应而导致的H2形成速率的强烈提升。由于H2的这种增强，通过D+ + H2反应在气相中更有效地形成HD。在尘埃温度低于150 K时，在尘埃颗粒上直接形成HD不能很好地与这种气相过程竞争。我们还推导了最新的H2和HD的晶粒表面形成的解析拟合公式，包括H和D的不同结合能。结果：粮食表面反应是至关重要的，在非常金属贫乏的环境中的H2和HD的可用性。在10^{-5}太阳的金属丰度以上，颗粒表面路线主导H2的形成，这又驱动气相中HD的形成。在尘埃温度高于150 K，实验室实验和理论建模表明，H2形成的晶粒被抑制，而HD形成的晶粒没有。

英文摘要：

Context: The HD and H2 molecules play important roles in the cooling of primordial and very metal-poor gas at high redshift. Aims: Grain surface and gas phase formation of HD and H2 is investigated to assess the importance of trace amounts of dust, 10^{-5}-10^{-3} Zo, in the production of HD and H2. Methods: We consider carbonaceous and silicate grains and include both physisorption and chemisorption, tunneling, and realistic grain surface barriers. We find, for a collapsing gas cloud environment with coupled chemical and thermal balance, that dust abundances as small as 10^{-5} solar lead to a strong boost in the H2 formation rate due to surface reactions. As a result of this enhancement in H2, HD is formed more efficiently in the gas phase through the D+ + H2 reaction. Direct formation of HD on dust grains cannot compete well with this gas phase process for dust temperatures below 150 K. We also derive up-to-date analytic fitting formulae for the grain surface formation of H2 and HD, including the different binding energies of H and D. Results: Grain surface reactions are crucial to the availability of H2 and HD in very metal-poor environments. Above metallicities of 10^{-5} solar, the grain surface route dominates the formation of H2, which in turn, drives the formation of HD in the gas phase. At dust temperatures above 150 K, laboratory experiments and theoretical modelling suggest that H2 formation on grains is suppressed while HD formation on grains is not.

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